Silver that is of very high purity has many important industrial applications including the manufacture of materials in the electronics industry and the preparation of silver nitrate for the photographic industry. Typically, the process for purifying silver relies on smelting, electrolysis or a combination of smelting and electrolysis.
Smelting is a pyrometallurgical process by which crude silver metal and or ionic silver containing compounds are heated in the presence of a flux. The silver ions are chemically reduced to silver metal and as a result of the extreme temperatures of the process, the silver metal melts and drops to the bottom of a reaction furnace. Meanwhile, base metals such as copper, iron, zinc, lead, nickel, etc., if present, remain oxidized and accumulate in the less dense slag waste stream. Finally, the molten silver is poured from the smelter and cooled in a casting. The process of smelting is inefficient with respect to mass balance and requires several days to complete. Furthermore, the precious metals, namely gold, palladium, platinum, rhodium, iridium, osmium and ruthenium remain in the purified silver and require separation during a secondary refining process such as electrorefining. Finally, sulfur and its congeners, selenium and tellurium, if not oxidized in the smelter, will be present in the purified silver and a second refining might be necessary.
Electrorefining also may be used to purify silver. Here, impure silver metal, typically pre-purified by smelting, is cast as an anode and is deposited as purified silver crystals at the cathode in an electrolytic cell. However, in this process silver ions are required to pass through an electrolyte, typically silver nitrate, that may contain an increased concentration of impurities from previously dissolved anodes. Frequent purges of the electrolyte are required to produce silver that is acceptable for use in applications that require high purity. Additionally, varying levels of sulfate, antimony, iron and sulfide are often found to be associated with electrorefined silver.
Hydrometallurgy is a less widely practiced method for refining silver. In practice, aqueous solutions of silver ions are reduced chemically to yield silver metal and, ideally, impurities are left in the aqueous phase. Depending on the identity of the impurities present in the silver matrix, it may be difficult to select a reducing agent of the appropriate strength. While the strength of the reducing agent or its propensity to reduce other species can be obtained from a table of thermodynamic electrochemical reduction potentials; it is not always possible to select a reducing agent that will selectively reduce silver ions to silver powder and leave all of the matrix impurities in a concentrated solid or in the aqueous phase.
The following are representative of the many references describing the purification of ionic silver or the preparation of silver powder from ionic silver solutions. For example, U.S. Pat. No. 5,749,940 describes a process for producing silver from de-copperized anode slime. The method relies on several leaching and separating steps and the one time addition of a reducing agent, dextrose, or hydrazine-a known carcinogen. GB 2,236,116 describes a process of producing silver powders of desired particle sizes from silver nitrate using formate and citrate in a one time addition. JP 61276907 describes the formation of monodisperse silver powders by treatment of aqueous silver nitrate in a gelatin solution with glucose. It also has been reported that crude silver chloride may be converted to silver oxide via the addition of caustic and subsequently reduced to silver metal by dextrose; but the final silver sponge requires secondary purification using electrorefining (Ackerman, John B.; Nordwick, Suzzann M.; Anderson, Corby G.; Krys. L. Ernst. Sunshine Min. Co. Kellog, ID, 83837, USA. Hydrometall. Proc. Milton E. Wadsworth Int. Symp., 4.sup.th (1993), 477-98. Editors: Hiskey, J. Brent; Warren, Garry W. Publisher: Soc. Min. Metall. Explor., Littleton, Colo.; Chpt. 30, pp 492-493).
RO 85165 describes a process of producing silver powder from silver nitrate using a one step addition of citric acid. Historically, metallic impurities are removed from aqueous silver ions by treating the matrix with a reagent to precipitate the impurities as metallic hydroxides. U.S. Pat. No. 2,543,792 describes the purification of aqueous silver nitrate with carbon, activated alumina and silver oxide. U.S. Pat. No. 2,614,029 describes the treatment of aqueous silver nitrate with silver oxide to maintain a pH of 6.1, the separation of the metal hydroxides which result from the solution, and the contact of the solution with a water insoluble porous solid absorbent such as activated alumina or magnesia. U.S. Pat. No. 3,554,883 describes a process of mixing silver nitrate with silver oxide to yield a pH of 5.1 to 5.8 causing the formation of a precipitate which is removed. The silver nitrate is subsequently treated with a second addition of silver oxide to yield a pH of 5.9 to 6.3 causing the formation of a second precipitate which is removed yielding a purified silver nitrate solution. Finally, U.S. Pat. No. 5,000,928 describes a process that yields ultra-pure silver nitrate from crude silver using a one time addition of sodium hydroxide to remove metal contaminates as hydroxy or oxy compounds; and a one time addition of sodium formate to yield silver metal which is subsequently converted to ultra-pure silver nitrate.
Even with all of the work which has been done around the purification of impure silver metal, there still remains a need for a less complex process to purify silver. Also needed is a process which has lower silver loss in the recovery stream. This must be accomplished while providing a silver purity level at least as high as that achieved by current technology.